MOTION 



49 



It is commonly believed that the primary source of the energy of 

 muscle is to be found in the interaction of several of its chemical con- 

 stituents. The potential energy here stored is transformed into kinetic 

 energy of the mechanical variety in accordance with the law of the 

 conservation of energy. Thus, the resting muscle represents an un- 

 stable system which may readily be transformed by the mere appli- 

 cation of a stimulus. The question now arises, how is it possible that 

 this explosive process leads to a shortening of the individual muscle 

 fibers as well as of the muscle as a 

 whole? Several explanations are 

 at hand, although the best of them 

 cannot be said to be absolutely 

 satisfactory. 



Weber ^ has claimed that the 

 contraction of muscle results in con- 

 sequence of the sudden alteration of 

 its elastic power, this change being 

 brought about by a chemical trans- 

 formation following in the wake of 

 the stimulus. These internal 

 chemical forces tend to cause varia- 

 tions in the elastic equilibrium of 

 the muscle, leading to a change in 

 its form. In accordance with the 

 view of Mayer (1845), muscle tissue 

 may be compared to a steam engine 

 which transforms the heat generated 

 by it into mechanical energy. En- 

 gelmann^ assumed later on that the 

 heat evolved results in a transfer of 

 molecules of water and a change in 

 the form of the muscle as a whole. 



Fig. 22, — Artificial Muscle. 

 The artificial muscle is represented 

 by the catgut string, m. This is sur- 

 rounded by a coil of platinum wire, w, 

 through which an electrical current may 

 be sent. The catgut is attached to a 

 lever, h, its fulcrum is at c. The cat- 

 gut is immersed in a beaker of water 

 at 50° to 55° C, and "stimulated" by 

 the sudden increase in temperature 

 caused by the passage of a current 

 through the coil. {Howell, after Engel- 

 mann.) 



This assumption has given rise to the 

 t30-called thermodynamic theory of mus- 

 cular contraction which is based upon 

 the observation that the contracting 

 fiber suffers an inversion of its elements, 

 le., the dark discs become more fluid and 



lighter in color, while the light discs become more compact and darker. But as 

 the width of the contracting portion of the fiber becomes greater, both bands must 

 be pushed out laterally and must therefore decrease in height. Engelmann then 

 assumed that the contraction of the fiber is caused by a rapid transfer of water 

 from the isotropic into the anisotropic substance under the influence of the chemical 

 energy set free in the form of heat. This imbibition with molecules of water tends 

 to impart a more oval or spherical shape to the individual contractile elements. 

 Later on, as the heat is dissipated, the water again returns into the light substance 

 and causes the fiber to relax. Engelmann has imitated this process of swelling 



1 Muskelphysik., 1846. 



2 pfliiger's Archiv, xi, 1875, 432 and xxv, 1881, 538. 



